Method of burning a particulate fuel and use of the method for burning sludge

ABSTRACT

PCT No. PCT/DK92/00039 Sec. 371 Date Aug. 12, 1993 Sec. 102(e) Date Aug. 12, 1993 PCT Filed Feb. 6, 1992 PCT Pub. No. WO92/14969 PCT Pub. Date Sep. 3, 1992.Method for producing hot drying gas by a burning flowable biological refuse in an incinerator which comprises a vertical cyclone furnace. Fuel together with primary combustion air is tangentially injected into the vertical cyclone furnace, and secondary combustion air and tertiary combustion air are injected into a throat. A cooled rotating ash scrapper is provided in the bottom of the vertical cyclone furnace and waste gas is conducted through the throat to a secondary combustion chamber in which an incineration of residuals takes place and from which a drying gas is removed. A combustion retarding gas is injected into the hottest area of the vertical cyclone furnace so that a sintering and the formation of slag is avoided.

FIELD OF THE INVENTION

The present invention relates to a method for production of hot dryinggases by incineration of fuel consisting of flowable biological refusein an incinerator comprising a primary combustion chamber in the form ofthe vertical cyclone furnace.

BACKGROUND OF THE INVENTION

In vertical cyclone furnaces, fuel may be injected tangentially in anupper half of the vertical cyclone furnace together with primarycombustion air, and secondary combustion air may be tangentiallyinjected in the same plane as that at which the primary combustion airis injected or higher in the primary combustion chamber. Ash isgenerally removed from a bottom area of the furnace by a rotating cooledash scrapper, and waste gas is transferred through an aperture in a topof the vertical cyclone furnace to a secondary combustion chamber.

In, for example, U.S. Pat. No. 4,398,477, two combustion chambers areprovided which consist of two cyclone furnaces arranged one above theother and connected through an opening with a reduced clearance, aso-called throat. The fuel, which consists of rice hulls, is blowntogether with the primary air in the lower vertical cyclone furnace, andthe waste gas is then burned in the upper cyclone furnace during theintroduction of additional combustion air through tangential nozzles.Through this arrangement an optimal incineration of the fuel isachieved, and the residual product in the form of ash can be removedfrom the bottom lower cyclone furnace by a cooled rotating ash scrapper.

In order to achieve optimal incineration of the fuel, the temperature inthe lower furnace is on the order of 1200° C. Such a high temperature isunfavorable in that, during combustion of biological fuels at thistemperature, relatively large amounts of poisonous nitrogen oxides (NOx)are formed.

In WO90/05272, a sludge drying apparatus is proposed wherein, forexample, sewage sludge is dried down to less than 10% water content in arotating dryer, after which the dried sludge is used as fuel in afurnace which delivers the thermal energy necessary for the rotatingdryer. However, it has proven to be almost impossible to incinerate thedried sludge in a normal cyclone furnace, with the reason being that thedried sludge and similar types of fuel vitrify to form a type of slagfilled with porous pores which have an insulating effect, while, at thesame time, the slag is highly viscous, thus rendering the removal of theslag impossible. Therefore, use is made in practice of other types offurnaces, for example, fluid-bed ovens, for the incineration of fuelswhich are aqueous or low energy content, such as, for example, driedbiological sludge. Furnaces of such a type are suitable only for largeamounts of fuel and require a long start-up time, and thus furnaces ofthis type are not suitable if they cannot be used in a continuousoperation. Moreover, this type of furnace demands a comprehensiveprocess regulation by specially-trained personnel.

Consequently, when it is necessary to dispose of refuse from smallertowns or urban areas, it is necessary to either use other methods of thedisposal of the biological sludge or to transport the sludge to largercommunal plants for incineration.

SUMMARY OF THE INVENTION

In accordance with the present invention, a method for the production ofhot drying gas by incineration of fuel consisting of flowable biologicalrefuse in an incinerator comprising a primary combustion chamber in theform of a vertical cyclone furnace includes tangentially injecting fuelin an upper half of the vertical cyclone furnace together with theprimary combustion air, and tangentially injecting secondary combustionair in the same plane as that at which the primary combustion air isinjected or higher in the primary combustion chamber. Ash is removedfrom a bottom area of the furnace by a rotating ash scrapper, and thewaste gas is transferred through a reduced aperture in a top of thevertical cyclone furnace to a secondary combustion chamber. A combustionretarding gas is tangentially injected into the ash separation area ofthe vertical cyclone furnace.

By virtue of the above noted features of the present invention, it ispossible to use a cyclone furnace for the incineration of dried flowablebiological refuse of the type which cannot otherwise be burned in acyclone furnace. The cyclone furnace has the great advantage that it isrelatively inexpensive to produce, that it is compact and results in anintensive combustion, and, most importantly, the cyclone furnace isquick and easy to start up. Consequently, cyclone furnace for theincineration of biological refuse does not need to operate continuously.

By arranging and controlling a furnace in accordance with the presentinvention, it is possible to burn bio-fuels of low calorific valuewithout the fuel sintering, which gives rise to the formation of slagand sintering the combustion zone. The combustion zone usually liesslightly below a middle of the furnace, with the reason being that thefuel will not ignite until the fuel has reached a good distancedownwardly toward the bottom and has achieved the ignition temperature.The introduction of combustion-retarding gas to the ash separation area,for example, oxygen-deficient air in the form of wet flue gas, willretard combustion so that this is less intense, and the sinteringformation is avoided. At the same time, a reduction in the formation ofNO_(x) is achieved because the surplus air is decreased and, providingthat the temperature is about 850° C., the CO formation can be held atan acceptably low level.

The waste gas formed by the controlled and retarded combustion is burnedafter the throat in a secondary combustion chamber which is merely alarge, brick-lined chamber in which the post-combustion takes place. Inorder to burn out the flue gas from the retarded combustion, it isnecessary for the secondary combustion chamber to be of a sufficientsize for the reduction of the CO content in the waste gas, and toprovide the waste gas with an adequate period of time in the secondarycombustion of, for example, 0.5-2 seconds.

In accordance with the present invention, tertiary combustion air isinjected tangentially and directly into the reduced aperture andsecondary combustion air is injected immediately below the reducedaperture, with the injections being effected with relatively high airvelocity, whereby sintering and the formation of slag at the throat isavoided, even during the use of dried biological sludge with low ashcontent. Moreover, a particularly good ash separation is achieved if thediameter of the throat is small relative to the diameter of the cyclonecombustion furnace, for example, a diameter which is less than one-halfof the diameter of the cyclone furnace, and if the air velocity isaround 60-100 m/sec.

In accordance with the present invention, an amount of thecombustion-retarding gas injected constitutes at least 10% of a totalamount of air injected into the vertical cyclone furnace, and is on theorder of about one-half of an amount of the primary combustion air, sothat it can be ensured that no sintering of the fuel with slag formationcan occur at any place within the combustion area in the cyclonefurnace. All ash/slag will fall to the bottom in the conical area of thefurnace, where by a cooled rotating ash scrapper, the ash/slag can beremoved from the furnace in the normal manner, for example, by an ashsluice.

According to the invention, a flowable fuel is used, with the fuel beingpulverized and screened so that at least 75% of the fuel has a particlesize of less than 1 mm, and a maximum particle size of 5 mm. By virtueof these last noted features, the operational reliability of the furnaceis increased so that a uniform and complete incineration of the fuel isachieved. The fuel is measured and screened so that it has the desiredparticle distribution. The smallest particles are ignited quickly andensure the combustion, while the large particles are held by thecentrifugal force in the periphery of the primary chamber untilcombustion has taken place.

If a poor fuel is used, for example, fuel with a high ash content orhigh water content, a subsidiary firing with an oil or gas injectioninto the secondary combustion air must be employed especially if thecalorific value of the fuel is less than 1700 kcal/kg. The subsidiaryfiring plant can be used in connection with the start-up of thecombustion furnace. However, when the fuel has a calorific value ofabout 1700 kcal/kg or higher, it is possible to maintain a constantcombustion of the fuel without any subsidiary firing.

Advantageously, according to the present invention, the secondarycombustion chamber has a volume which is at least sufficient for thewaste gases to exist therein for at least 0.5 sec. by this construction,a complete combustion of the waste gas is achieved so that the COcontent is burned to CO₂, and a suitably low CO content is achievedwithout any significant formation of NO_(x).

In order to ensure that the sintering of the fuel cannot take place atany point in the furnace during combustion and at no point in thefurnace is there any formation of viscous slag, according to theinvention, the injection of combustion air and the combustion-retardinggas is effected in such a manner that the temperature does not exceed950°-1000° C. at any point in the vertical cyclone furnace. Thecombustion throughout the entire primary combustion chamber is aso-called dry or non-slagging combustion, with the only waste productsbeing ash and flue gas, and, where the ash is of such a consistency thatit can be removed without problems by a commonly known rotating ashscrapper.

It has been experimentally determined in connection with the burning offuel which consists solely of biological refuse in the form of driedsludge, the best incineration of the fuel is achieved when an amount ofthe injected combustion-retarding gas is on the order of about one-halfof an amount of the primary combustion air, and an amount of thesecondary combustion air is of the same order as the amount of theprimary combustion air.

The method of the present invention may be used for the incineration inthe vertical cyclone combustion furnace of biological sludge with awater content of less than 25%, with a hot drying gas being used forpredrying the biological sludge in a drying plant. Moist drying air fromthe drying plant may be recirculated to the incinerator and used as thecombustion-retarding gas in the vertical cyclone furnace.

Moreover, the method of the present invention may be used in anarrangement where the vertical cyclone combustion furnace is arranged ina drying plant for aqueous masses such as, for example, biologicalsludge in a manner more fully described in connection with WO90/05272.

However, as can well be appreciated, the method of the present inventioncan naturally be used in connection with the burning of other forms ofbiological fuel.

BRIEF DESCRIPTION OF THE DRAWING

The method of the present invention will now be described in more detailwith reference to the accompanying drawing, wherein:

The Single Figure of the drawing is a schematic view of an incineratorincluding a vertical cyclone furnace connected to a secondary combustionchamber via a throat constructed in accordance with the presentinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

As shown in the Single Figure of the drawing, according to the presentinvention, an incinerator for bio-fuels, for example, dried sludge,comprises a primary combustion chamber in the form of a vertical cyclonefurnace 2, a throat 5 and a secondary combustion chamber for subsequentincineration of the waste gas from the cyclone furnace 2.

In the bottom of the conical part 12 of the cyclone furnace there isprovided a rotating ash scraper 11 which is air-cooled in the normalmanner, and which scrapes the ash 14 out through a not-shown ash sluice10 or an ash conveyor with product lock.

The top of the secondary chamber 3 is arranged for the removal of thehot waste gas 4, which, for example can be used directly in a rotarydrier as described in more detail in International Patent ApplicationNo. PCT/DK89/00246 (W090/05272), and to which reference is made to allextent in connection with the use of the hot drying gas 4.

The primary air/fuels 6 is blown in through tangential injectionnozzles. The fuel is bio-fuel, e.g. dried sludge, supplied from a sourceF as explained in more detail in the above-mentioned internationalapplication. The dried bio-fuel in the form of sludge is dried down toless than 15%, preferably 10%, water content, pulverized in a mill M andscreened through, for example a 5 mm sleeves. The main part of the fuel,for example, at least 75%, has a particle size of less than 1 mm, andthe maximum particle size due to the sieve is 5

At the same level, or possibly slightly above in the cyclone furnacethan that at which the primary air is injected, the secondary air 7 isinjected through a series of tangential nozzles, and tertiary air 8 isblown into the throat 5 itself, similarly through a number of tangentialnozzles. A modest amount of combustion air is also injected through thecooled ash scraper 11, in that cooling-air is introduced into thecombustion chamber through openings in the ash scraper 11.

At some distance down in the cyclone furnace, preferably at around themid-point or immediately below, the injected air/ fuel 6 will be ignitedand will burn. In order to control and dampen the intensity of thecombustion, so that the fuel does not sinter and give rise to theformation of slag in the combustion zone, combustion-retarding gas 9 isinjected directly into the combustion zone via tangential nozzles in thedirection of rotation for the combustion.

The combustion-retarding gas is air with reduced oxygen content and/orwith high moisture content, so that the oxygen content of the air isreduced approx. 30-50% in relation to normal atmospheric air, and theair has a temperature in on the order of 100°-200° C., preferably 150°C. The air, for example, is recirculated drying air with a temperatureof approx. 150° C. from the rotary dryer in the above-mentionedinternational application. The amount of combustion-retarding air 9 canbe set once and for all, depending on the capacity of the furnace.Primary air, secondary air and tertiary air is also set once and forall, similarly depending on the capacity of the furnace. The temperatureof the furnace is controlled at approx. 850° C. If the temperaturefalls, the amount of injected fuel is increased. If the temperaturerises, the amount of injected fuel is reduced. There is hereby achieveda very simple and reliable form of control, which at the same timeensures that the temperature does not exceed 950°-1000° C. at any pointin the primary chamber.

With an incinerator of the type described, and controlled as explainedabove, a cyclone combustion 13 is achieved whereby with the use ofgravitation and the special form of injection for the combustion air,the combustion takes place in a downwardly-directed spiral movement asshown in the drawing, and where the waste gas, similarly sketched in thedrawing, is transferred via the throat 5 to the post-combustion chamber3 for incineration. The post-combustion chamber 3 is at least of thesame size as the cyclone furnace, but will normally have a volume whichensures that the period of time for which the waste gases are in thechamber is at least 0.5 seca.

The following table shows a series of different values for incineratorscontrolled according to the invention and used in connection withrecirculated waste gas (drying air) and biological fuel from a rotarydryer as disclosed in the above-mentioned international application.

    ______________________________________                                        Type              30-190                                                      ______________________________________                                        Evaporation kg/h   500-3,200                                                  Person equivalents                                                                              30,000-190,000                                              Wet sludge t/week                                                                               63-400                                                      Ash t/week        6-36                                                        Furnace effect MW                                                                              0.5-2.8                                                      Primary air %    30                                                           Secondary air %  30                                                           Tertiary air %   15                                                           Scraper air %    10                                                           Recirculated air %**)                                                                          15                                                           Waste gas %*)    100                                                          ______________________________________                                         Prerequisites: 60 g solids per person equivalent per 24 hours; the dried      sludge has 20% solids, of which 40% is ash. Operational time per week is      100 h.                                                                        *)This drying air, which, for example, is used in a rotary dryer as           disclosed in the abovementioned international application, has a              temperature of approx. 850° C. and a NOx content of less than 100      ppm.                                                                          **)The air has a temperature of 100-150° C., an oxygen content of      10-12% and a moisture content of 0.4 kg water per kg dry air.            

At the start-up of the incinerator, oil or gas, for example, N-gas, isintroduced in the secondary air 7 by nozzles. These nozzles (not shown)are also used for subsidiary firing if the fuel has a calorific value ofless than 1700 kcal/kg.

What is claimed is:
 1. Method for production of hot drying gas byincineration of fuel consisting of flowable biological refuse in anincinerator comprising a primary combustion chamber in the form of avertical cyclone furnace, the method comprising the stepsof:tangentially injecting the fuel in an upper half of the verticalcyclone furnace together with primary combustion air, tangentiallyinjecting secondary combustion air in the same plane as that at whichthe primary combustion air is injected or higher in the primarycombustion chamber, removing ash from a bottom area of the furnace by arotating cooled ash scrapper, transferring waste gas through a reducedaperture in a top of the vertical cylindrical furnace to a secondarycombustion chamber, and tangentially injecting combustion-retarding gasinto an ash separation area of the vertical cyclone furnace.
 2. Methodaccording to claim 1, further comprising the step of injecting tertiarycombustion air tangentially and directly into the reduced aperture, andinjecting the secondary combustion air immediately below the reducedaperture, wherein said injections are effected with relatively high airvelocity.
 3. Method according to one of claims 1 or 2, wherein an amountof said combustion-retarding gas injected constitutes at least 10% of atotal amount of air injected into the vertical cyclone furnace, and ison the order of one-half of an amount of said primary combustion air. 4.Method according to claim 3, wherein the fuel is a flowable fuel, andwherein the fuel is pulverized and screened so that a least 75% of thefuel has a particle size of less than 1 mm, and a maximum particle sizeof 5 mm.
 5. Method according to claim 4, further comprising the step ofeffecting a subsidiary firing with oil or gas injection into thesecondary combustion air if a calorific value of the fuel is less than1700 kcal/kg.
 6. Method according to claim 1, wherein the secondarycombustion chamber has a volume at least sufficient for the waste gasesto exist therein for at least 0.5 sec.
 7. Method according to one ofclaims 1 or 2, wherein the injection of combustion air andcombustion-retarding gas is effected in such a manner that thetemperature does not exceed 950°-1000° C. at any point in the verticalcyclone furnace.
 8. Method according to one of claims 1 or 2, wherein anamount of injected combustion-retarding gas is on the order of aboutone-half of an amount of primary combustion air, and wherein an amountof the secondary combustion air is of the same order as the amount ofthe primary combustion air.
 9. A method according to one of claims 1 or2, wherein the vertical cyclone combustion furnace incineratesbiological sludge with a water content of less than 25%, hot drying gasis used for a predrying the biological sludge in a drying plant, andwherein moist drying air from the drying plant is recirculated to theincinerator and is used as the combustion-retarding gas in the verticalcyclone furnace.
 10. Method according to one of claim 1 or 2, whereinthe vertical cyclone furnace is arranged in a drying plant for aqueousmasses.
 11. Method according to claim 10, wherein the aqueous massesinclude biological sludge.